Work over the pat 17 years has reclassified Blastocystis hominis as a protozoan, removing it from the yeasts where it has languished for 70 years. B. hominis now is established firmly as a member of the Sporozoa, and intestinal parasite and pathogen of man and primates. This protozoan is now a subject of national and international interest to parasitologists and physicians as a cause of diarrhea in man. Treatment is usually with metronidazole. Trimethoprim- sulfamethoxazole also is used successfully. Ultrastructural evidence has proven that B. hominis mitochondria are indeed typical mitochondria. It has been stated that the B. hominis organelle was not a mitochondrion since the tubular cristae appeared to open to the outside; Dr. J. Muller, using special techniques, provided clear photographs of the enveloping outer membrane. Data collected over 1986-87 include: 1. Positive correlation between added cytochrome C and mitochondria numbers, and uptake of carbohydrate metabolites; 2. Purification and partial analysis of a polysaccharide slime material synthesized by Blastocystis.